Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -1Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Advanced Design Flexibility WorkshopAdvanced Design Flexibility WorkshopSession 6Session 6Jack Broz, PE, HR GreenMay 5-7, 2010
Advanced Design Flexibility WorkshopAdvanced Design Flexibility WorkshopSession 6Session 6Jack Broz, PE, HR GreenMay 5-7, 2010
Selecting Design SpeedSelecting Design Speed
Conventional Approach
• Classification required by Federal lawq y• General Categories:
– Arterial– Collector– Local
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -2Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Functional Classifications
Changing Road Classification
• Planning level exercise not normally a• Planning level exercise – not normally a project stage consideration
• Long-range implications for overall road network
• Normally subject to established reviewNormally subject to established review cycle
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -3Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Keeping the Big Picture in MindThe network works only if there is a “local”
functioning network!“regional”
And vice versa!
Project Development Process
Once the function and context of the roadwayhas been systematically considered, the choice ofDesign Speed follows, which greatly influencessubsequent design parameters
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -4Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Role of Speed in Transportation• Speaks to the convenience and economy of
a facility in terms of time and cost
• Influenced by 5 general conditions:– Highway Physical Characteristics
– Roadside Interference
– Weather Conditions
– Traffic Interaction
– External Speed Limitations
2007 Speed-Safety Facts*• 106 Fatal Crashes out of 463 Crashes
• 3727 Injury Crashes out of 24,978
• 7085 Property Damage Crashes out of 56,064
f li i ( ) f• 28% of Fatalities (2001-2005) - One of Mn/DOT’s seven critical emphasis areas.
*From the Minnesota Department of Public Safety
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -5Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Changes to Design Speed Concept
CurrentInterstate EraPre-WWII
Assumed Design Speed (Barnett 1936):“The maximum reasonably uniform speed which would be adopted by the faster driving group of vehicle operators, once clear of urban areas”
Changes to Design Speed Concept
CurrentInterstate EraPre-WWII
Design Speed (AASHO 1938):“The maximum approximately uniform speed which probablywill be adopted by the faster group of drivers but not, necessarily, by the small percentage of reckless ones.”
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -6Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Changes to Design Speed Concept
CurrentInterstate EraPre-WWIIAssumed Design Speed (AASHO 1940):“The Assumed Design Speed selected for a highway is determined by consideration of the topography of the area traversed, economic justification based on traffic volume, cost of right-of-way and other factors, traffic characteristics, and other pertinent factors such as aesthetic considerations.”
Changes to Design Speed Concept
CurrentInterstate EraPre-WWIIAASHO (1954, 1965):( , )“The speed determined for design and correlation of the physical features of a highway that influence vehicle operation. It is the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern.”
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -7Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Changes to Design Speed Concept
CurrentInterstate EraPre-WWII
AASHO (1973):“The maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern.”
Changes to Design Speed Concept
CurrentInterstate EraPre-WWIIAASHTO (1984, 1990, 1994):( )“The maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern. The assumed design speed should be a logical one with respect to the topography, the adjacent land use, and the functional classification of highway.”
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -8Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Changes to Design Speed Concept
CurrentInterstate EraPre-WWII
AASHTO (2001, 2004) and MUTCD (2000, 2003):“Design Speed is a selected speed used to determine the various geometric design features of the roadway.”
Target Operating Speed
Concept of Desired/Target Speed
– “…desirable speed at whichvehicles should operate on athoroughfare in a specificcontext.” (ITE)
– “…the desired operating speedalong a roadway Analong a roadway. Anappropriate target speed shouldbe determined early in theproject development process.”(FHWA)
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -9Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
AASHTO DESIGNSPEED
Where We Are Today
ACTUALOPERATING
SPEED
POSTEDSPEED
TARGETOPERATING
The Goal for the Future
OPERATINGSPEED
ACTUALOPERATING
SPEED
RATIONALPOSTEDSPEED
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -10Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
From the MassHighway Guide
Making the Choice• Read between the…Tables!
GB Chapter 3, Page 281:“The selected design speed serves to keep all
elements of design in balance”GB Chapter 5, Page 406:“A decrease in design speed along the roadA decrease in design speed along the road
should not be introduced abruptly, but be extended over a sufficient distance to allow the driver to adjust and make the transition to the slower speed.”
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -11Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Consider Design Exception for Design Speed (Lake St)
Making the Choice:“A Minor” Arterial
Case Study: US 151, WI
Saturday morning
35 mph Posted Speed
25 mph travel speed
3.7 mile trip length
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -12Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Case Study: US 151, WI
• Criteria for setting speed limits
Engineering for Speed Managementg p
• Design of self-enforcing roads
• Speed control on curves
• Variable speed limits
• Onboard technology
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -13Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Self-Enforcing/Self-Explaining Roads• Important Design Focus Areas
R l A– Rural Areas• Many types/functions of 2-lane
rural roads• Make the effort to “get to know”
the subject road
– Transitions• Undeveloped to Developed• Undeveloped to Developed• Developed to Urban Core
– Curves• First curves after long tangents• Comparably more restrictive
curves
Rural: 65 mphRural: 65 mph
Urban: 45 mphUrban: 45 mph
TransitionTransition
TransitionTransition
Urban Core: 35 mphUrban Core: 35 mph
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -14Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
• Transition elements– Curb & Gutter– 4’ Mountable Raised Median– Roadway Lighting– Community Gateway Signage
• Transition elements– Drop Shoulders– 4’ Non-Mountable Median– Decorative Roadway Lighting
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -15Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
• Historic Contextual Elements– Plaza with benches and plantings– Narrowed Lanes– Decorative Roadway Lighting– Mini-Park with historic plaque
• Historic Contextual Elements– Plaza with benches and plantings– Narrowed Lanes– Decorative Roadway Lighting– Mini-Park with historic plaque
Design Element
Relationship to Design SpeedDi t I di t Oth
Major Geometric Elements Affected
Element Direct Indirect OtherSight Distance X
Horiz Curvature X
Superelevation XGrade X
Vert Curvature XClear Zone X
Road X-Section* X X X
* ‘Road X-Section’ includes lane width, shoulder width, use of C&G, etc.
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -16Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Research on Design Speed Issues
S ht l ti b t• Sought correlation between Design Speed, Operating Speed and Posted Speed
• Provides equations to assist in predicting operating speeds
• Makes recommendations for best practice for selecting Design Speed
Source: NCHRP Report 504
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -17Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Suburban/ Urban SpeedsTable 24 Percentile speed that equals posted speed by area type and posted speed How do you select
Design Speed?
Area Type Speed Limit(mph) Percentile at or below Given Speed*. Number of
Speed Limit Speed Limit Speed Limit Sites
Plus 5 mph Plus 10 mph
Suburban/ Urban 25 42 77 94 730 28 64 86 1935 22 62 90 2340 32 68 92 2545 37 70 90 1550 43 76 95 955 48 80 95 6
Source: NCHRP Report 504
Vehicle Speeds and PedestriansUK: Department of
Environment, Transport, and the Regions, (DETR)
Florida, 1993-1996; pedestrians in single-vehicle crashes
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -18Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
~90% Fatality Rate~50% ~10%
Source: NCHRP Report 504
• Strongest relationship between Posted and Operatingi l l i b i d O i d
Noteworthy Report 504 Findings
• Little correlation between Design and Operating speeds except at tight horizontal curves or short vertical curves
• Other speed-influential variables– Access Density– Median (presence, type)– On-street ParkingOn street Parking– Pedestrian Activity Level
• No evidence that presence of C&G results in lower Operating Speed
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -19Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Process for Selecting Design Speed• Speed Prediction Feedback Loop
– Design a preliminary alignment based on a selected design speed
– Estimate operating (85th percentile) speeds for the preliminary alignment
– Check for large differences in operating g p gspeeds at curve locations and grade changes
– Review against desired operating speed range– Revise the alignment to reduce these
differences to acceptable levels
Land Use TopographyFunctional Classification
Engineering for Speed Management
Anticipated Operatingor Target Speed
Design Speed Determination
Determine Geometric and Cross-Section Elements
Check for Consistency Predict OperatingCheck for Consistency among Design Elements
Final Design and Construction
Set Speed LimitObserve ActualOperating Conditions
Predict Operating Conditions
Session 6Session 6Selecting Design Speed
Advanced Design Flexibility Workshop
May 20106 -20Minnesota Department of Transportation
University of Minnesota Center for Transportation Studies
Exercise
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